I think it is macOS Mammoth.
Mammoth Mountain was part of the Sierra National Forrest. Eastern portions of Sierra National Forest became the Inyo and Toiyabe National Forests.
Apple used
"Yosemite" (National Forrest) for OS X 10.10.x,
"El Capitan" (in the Yosemite National Forrest) for OS X 10.11.x
and
"Sierra" (National Forrest) for macOS 10.12.x.
macOS 10.14.x is probably macOS "Sequoia".
Mammoth Mountain was part of the Sierra National Forrest. Eastern portions of Sierra National Forest became the Inyo and Toiyabe National Forests.
Apple used
"Yosemite" (National Forrest) for OS X 10.10.x,
"El Capitan" (in the Yosemite National Forrest) for OS X 10.11.x
and
"Sierra" (National Forrest) for macOS 10.12.x.
macOS 10.14.x is probably macOS "Sequoia".